Radiation-curable compositions have been used to provide coatings for optical fibers. Optical fibers are generally coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which is in direct contact with the glass is called the inner primary coating and the overlaying coating(s) is called the outer primary coating.
The inner primary coating is usually a relatively soft coating providing environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending. Microbending in the coated fiber can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating(s), which is on the exposed surface of the coated fiber, is typically a relatively harder coating designed to provide a desired resistance to physical handling forces, such as those encountered when the fiber is cabled.
Inner primary radiation-curable compositions typically comprise a radiation-curable oligomer and a reactive diluent. The oligomer has a relatively high molecular weight, which allows for a certain integrity and toughness of the composition when cured, but which also, in conventional compositions, raises the viscosity of the compositions to a level that is unacceptable in fiber coating processes. Accordingly, substantial amounts of reactive diluent are added to bring the viscosity down to acceptable levels.
However, a drawback of reactive diluents is that they can increase the glass transition temperature of inner primary coatings, often resulting in an undesirable increase of the tensile modulus, in particular at low temperatures. This increase in modulus can result in attenuation of the signal transmission capability of the optical fiber.
A further drawback of the relatively low molecular weight reactive diluents is the release of volatile components during coating processes. Accordingly, it is an object of the present invention to provide a radiation-curable composition comprising a low amount of reactive diluents while still exhibiting a sufficiently low viscosity for coating applications.
Another aspect of the invention is providing a radiation curable composition comprising a low amount of reactive diluents and exhibiting, after cure, a low glass transition temperature.